1. Field of the Invention
The present invention relates to an image processing apparatus and its method, a computer program and a storage medium. For example, the present invention relates to image processing of a copy produced by using a scanner.
2. Description of the Related Art
An apparatus which realizes by itself a plurality of functions as a copy machine, a printer, and a facsimile machine (FAX) is referred to as a multifunction peripheral (MFP). A scanner which is mounted on the MFP or the like includes an optical system for introducing light reflected from a document positioning plate by scanning to a solid-state image sensor such as a charge coupled device (CCD).
The optical system has fixed resolving power and can convert a contrast into a numerical value by calculation. In a scanner which has high resolving power, even a small character can distinctly be scanned. Whereas, in a scanner which has low resolving power, even a large character is indistinctly scanned.
In a conventional scanner which is mounted on an MFP or the like, target resolving power is determined using a contrast converted into a numerical value and adjusts the scanner so as to fit thereto. However, resolving power may vary under an influence of a user's usage, change with the passage of time, or the like. An amount of the variation is changed depending on an environment such as temperature and humidity, a number of activating times, and the like. Thus, in each scanner, the amount of the variation is different.
Thus, for example, in an apparatus which mounts a scanner unit configured to read a front side of a document and a scanner unit configured to read a back side thereof, resolving power reading the front side and resolving power reading the back side may be different. When resolving power is different, a problem arises in which a scanning result of characters and halftone dot portions is different between a scanner for reading the front side and a scanner for reading the back side even if the same document is read.
Further, even if the same scanners are used, resolving power may be different in a plane due to adjustment of the scanner. Accordingly, in a conventional technique, a chart which has fixed resolving power as indicated in a chart 101 of FIG. 1 is scanned to detect abnormality of resolving power of a scanner. Then, a scanning result is compared with a specified value to correct the resolving power of the scanner using the detected value (e.g., refer to Japanese Patent Application Laid-Open No. 2000-349966).
However, in Japanese Patent Application Laid-Open No. 2000-349966, to detect resolving power of a scanner, a chart which has specified resolution must be prepared. In order to generate a chart which has specified resolution, a high-performance printing machine is required. Further, there has been a problem that the chart itself is expensive. Furthermore, a user who uses a scanner needs to store the chart under a strictly controlled condition to prevent folding or soiling of the chart.
Further, a phenomenon occurs in which appearance of a highlight portion of a halftone dot document is different depending on resolving power of a scanner for a reason described below. The highlight portion refers to a light color data part. However, when a contrast converted into a numerical value is calculated using the chart 101 to execute processing for adjusting the resolving power, it is difficult to correct the resolving power when the contrast value and the actual appearance are not correlated. Particularly, if a degree of accuracy of the chart is low such that a line width of a line printed on the chart narrows, it is highly likely that correlation between the contrast value and the actual appearance is not established. As described above, when correction is executed using resolving power converted into a numerical value, there has been a problem that a result of the resolving power correction is different depending on accuracy of a chart.